Loading the drive axle

UK PROFESSIONAL DRIVERS FORUMS THE UK PROFESSIONAL DRIVERS FORUM (INTERACTIVE)
1

This applies mainly to artics and lorry/drag, but feel free to discuss rigids too.

During another discussion in a different part of the forum, i mentioned about trying (where possible) to get as much weight as is reasonable over the drive axle, especially when conditions deteriorate.

Another poster mentioned that there is a school of thought that a heavily loaded drive axle and lightly loaded trailer axles could help provoke a jack knife situation.

The thread in question hasn’t provoked any responses to this so i thought i’d throw it open to you lot.

My opinion is that i find an artic or W&D load weighted in favour of the drive axle is invariably best for overall vehicle stability, grip, handling, braking control etc, especially when things get slippy and loaded as such a jack knife is less likely.

Whats your opinion and experiences, its not a row or an argument, far from it, i’d be genuinly interested to hear other views.

Things might have been different 30 years ago before we had load sensing valves and you could easily have a fully locked trailer if the axles were too light and fully braked, but i’m specifically asking about now assuming you drive modern kit with anti lock brakes.

2

Personally I’d agree with you, as much of the weight as possible over the drive, without overloading and assuming it won’t cause an effect of lightening the front steer. Most modern trailers should be able to adjust the brakes accordingly. Too light on the drive can imo cause it to skip and spin in tricky conditions, hence RWD cars such as BMWs struggle so much in winter.

3

I don’t know about the bendy bit in the middle, but without my Manitou on board, my old girl will not reverse up even gentlest of inclines. By gentle I mean, as close to a billiard table with one leg placed on a beer mat.

4

I reckon that the overwhelming majority of jacknifes are caused when the drive axle breaks traction under braking, which tends to support the theory that more weight on it is better.

I’ve nearly jacknifed at low speed when loaded and turning whilst moving downhill on ice, but that was my stupidity for putting myself in that situation in the first place and could easily have been avoided.

I suppose in the old days when the trailer brakes were either on or off (or so it felt) you could possibly have the trailer whip around you in extreme circumstances, but I reckon with todays stability programmes, ABS etc you’d need to try really hard to do that. Still, I know some drivers who could manage it easily!

5

Juddian:
Another poster mentioned that there is a school of thought that a heavily loaded drive axle and lightly loaded trailer axles could help provoke a jack knife situation.

were they talking about a hollywood jack knife or the normal - back of the unit whips round?

get plenty on the drive for me

6

A lot of w&d drivers don’t give any thought about the trailer load position over loading the drive or rear axle of the rigid especially on multidrop starting from the back of the trailer .

7

Found this page, and by the way it explains things, these are on vehicles with out ABS and load sensors, which is irrelevant to the modern vehicles that we have the pleasure to drive now.

Most jackknifes now are caused by the way the truck is being driven, and not caused by the braking system.
Another poster mentioned that there is a school of thought that a heavily loaded drive axle and lightly loaded trailer axles could help provoke a jack knife situation.
That example would more than likely cause trailer swing, but only if the trailer ABS system is faulty.
I am pretty confident that the modern truck I am driving now would not jackknife regardless of the way it is loaded under braking compared to trucks I drove pre ABS.
reality is, the jackknife myths come from pre ABS days when a bit of rain would have A1 being blocked by jackknifed artics, think most these days start like this example on youtube

which isn’t a truck induced jackknife.

8

Juddian:
This applies mainly to artics and lorry/drag, but feel free to discuss rigids too.

During another discussion in a different part of the forum, i mentioned about trying (where possible) to get as much weight as is reasonable over the drive axle, especially when conditions deteriorate.

Another poster mentioned that there is a school of thought that a heavily loaded drive axle and lightly loaded trailer axles could help provoke a jack knife situation.

The thread in question hasn’t provoked any responses to this so i thought i’d throw it open to you lot.

My opinion is that i find an artic or W&D load weighted in favour of the drive axle is invariably best for overall vehicle stability, grip, handling, braking control etc, especially when things get slippy and loaded as such a jack knife is less likely.

Whats your opinion and experiences, its not a row or an argument, far from it, i’d be genuinly interested to hear other views.

Things might have been different 30 years ago before we had load sensing valves and you could easily have a fully locked trailer if the axles were too light and fully braked, but i’m specifically asking about now assuming you drive modern kit with anti lock brakes.

It would be interesting to find out why anyone would think that a light drive axle would reduce the possibility of a jacknife situation and vice versa.Which still leaves the opposite problem of an overloaded drive axle but which doesn’t create the same jacknife risk.Both of which problems are probably more likely with the close coupled drawbar configuration.IE too much weight behind the centre trailer axles or too much in front.The former lifting weight off the drive axle and the latter putting too much onto it.The issue of a light drive axle applies to lesser extent in the case of artics because the trailer axles are set further back.With the best configuration in that regard being the A frame drawbar in which trailer weight distribution has no effect whatsoever on the prime mover’s axle weights.

9

You see things loaded on flat-beds sometimes with whatever it is over the trailer axles but nothing on the front.
I dunno why but maybe if there’s some weight over the trailer wheels the brakes would be less likely to lock up so the trailer brakes would be more effective… Less chance of the trailer pushing the unit forward when turning
Then again, if there was weight over the drive axle then those brakes would work better so less chance of the trailer being able to push the back of the unit round

Hmmmmm

10

daffyd:
You see things loaded on flat-beds sometimes with whatever it is over the trailer axles but nothing on the front.
I dunno why but maybe if there’s some weight over the trailer wheels the brakes would be less likely to lock up so the trailer brakes would be more effective… Less chance of the trailer pushing the unit forward when turning
Then again, if there was weight over the drive axle then those brakes would work better so less chance of the trailer being able to push the back of the unit round

Hmmmmm

In general the worst situation on an artic or close coupled outfit is too much weight behind the centre line of the trailer axles.Which ‘lifts’ weight off of the unit/prime mover drive/s/rear axle bogie and puts it on the rearmost trailer axle.In addition to which it is always best to have as much as possible on the unit/prime mover drive/s/rear bogie.Although too much will not only overload the drive/s/bogie but can also lift it off of the steer axle.While the issue of twitchy handling and potential jacknife situation caused by a light drive/s/bogie on a unit or prime mover of a close coupled outfit isn’t just related to braking or lock ups.It is a matter of forward directional momentum of the heavy trailer overcoming the lateral grip of the the relatively lightly loaded drive/s/bogie tyres caused by the sideways action of the single point of articulation in any type of overrun situation.IE artics and close coupled outfits are extremely sensitive to weight distribution in terms of their axle weights,handling and stability.Moreso in the case of the close coupled drawbar.While the 3 + 2 or more A frame drawbar outfit is about as good as it gets.

11

is putting the load over the trailer wheels maybe an old school thing when there was no ABS etc?
less likely to lock the trailer wheels?

12

I would like to bring rigid 6 x 4 's into this debate.Loading the drive axles too heavily on these vehicles results in really bad understeer especially on slippery surfaces.This type of truck naturally wants to go straight on all the time because of the 8 tyres on two fixed axles pushing 2 that are steering.You do have plenty of traction though to get up a farm drive etc.
If the load is biased to the front you get positive but heavy steering,poor traction and a choppy,uncomfortable ride so it’s all about a balanced load distribution.
I think the same applies to a tractor unit with a sliding fifth wheel.
Agree with most of the other posts ,I would definitely bias the load onto the tractor unit with due consideration to axle weights whether on ABS or not,its surprising how easy it is to lose traction especially when turning a tri-axle trailer.

13

Just load all the Wight on the trailer axles go down the road in the snow and ice brake very hard and see what happens with little or no load on the drive axle

There may be a very big bang tail waging the dog get as much Wight on the drive as you can in snow and ice

14

offsetphil:
I would like to bring rigid 6 x 4 's into this debate.Loading the drive axles too heavily on these vehicles results in really bad understeer especially on slippery surfaces.This type of truck naturally wants to go straight on all the time because of the 8 tyres on two fixed axles pushing 2 that are steering.You do have plenty of traction though to get up a farm drive etc.
If the load is biased to the front you get positive but heavy steering,poor traction and a choppy,uncomfortable ride so it’s all about a balanced load distribution.
I think the same applies to a tractor unit with a sliding fifth wheel.
Agree with most of the other posts ,I would definitely bias the load onto the tractor unit with due consideration to axle weights whether on ABS or not,its surprising how easy it is to lose traction especially when turning a tri-axle trailer.

8 wheelers can be as bad for wanting to push straight on, tbh a 6x4 can have similar problems.

Still wouldn’t be without a double drive in construction work though.

15

Some interesting points there, it seems all of us like having most weight over the drive wheels where possible, and vice versa i don’t think anyone would want a load sitting over the trailer axles only.

Many thanks for the input, sometimes when someone asks a pertinent question about something you do almost automatically it makes you wonder.

One really memorable trip was a snowy winters night umpteen years ago, lorry and A frame drag, empty box on the lorry, heavily loaded box on the trailer (different size boxes, no swap possible), lots of lorry tail out slides but fast as you corrected it one way the A frame sent you another, 30 odd years later i wouldn’t have taken it like that.

daffyd:
You see things loaded on flat-beds sometimes with whatever it is over the trailer axles but nothing on the front.
I dunno why but maybe if there’s some weight over the trailer wheels the brakes would be less likely to lock up so the trailer brakes would be more effective… Less chance of the trailer pushing the unit forward when turning
Then again, if there was weight over the drive axle then those brakes would work better so less chance of the trailer being able to push the back of the unit round

Hmmmmm

I’ve quoted Daffyd here because he raises an interesting point, for general braking maybe more weight on the trailer would help the trailer pull the tractor up…going back a lifetime, Scanias used to have the trailer only brake handle on the 110/111s that i drove, you could use well loaded trailer to pull you up straight just using the exhauster and trailer handle without touching the footbrake at all, i liked that system because it let the driver decide how to brake, but haven’t seen a trailer only brake fitted to a lorry for 30 years now.

With modern systems though i’m more than happy to carry on loading towards the drive axle, especially with its grip benefits come the white stuff.

Offsetphil, good point that, my boss at the time hired in a Bedford TM 6 wheeler with a Rolonoff hooklift on it, the wheelbase was far too short so when you had a 40 yard bin on the back the bin sat square over both drive axles, had a dickens of a job to get that thing round corners at all loaded and i have no idea what weight distriubution was like, all over the shop i suspect, anything over 50mph with a full bin she’d start to swaying like a caravan…however, on the tip with all that weight back, unstoppable.

Other than that one i loved 6 and 8 wheelers for their stability, would gladly have another.

On the subject of sliding 5th wheels, most blokes i see just shove 'em to the very back and leave them there, spent several weeks perfecting mine by regularly using a customers axle weigher to get it exactly right.
Mine was the only one out of 6 identical lorries that didn’t have the 5th wheel fully back, and several drivers who used mine commented how much more stable it is than the others.

16

get the weight on the drive wheels ive had 20ft containers in the rain and my wheels have been spinning,
I don’t want to get technical, their are so many variance its not hard to tell when you are south of the law.
if snow or ice the more the merrier trust me in snow the last thing the police are worried is your axel weight.

17

I must admit I have not read all the replies but a 6X2 in winter or on slippy ground with the tag axle lifted has much more grip than with the tag axle down. Forget the regulations. Do what is best for you.

18

It was reported in CM a while ago that VOLVO had reinvented the wheel or rather had ‘invented’ a seperate brake that brought the trailer brakes
on first…this would avoid jacknife situations…that would be the brake we had over 30 years ago,great for OD’s running someone elses kit,use
their brakes…but it also revealed that, according to VOLVO their modern electronic braking systems ■■■■ hence the need for another system so
if your driving a VOLVO be careful even VOLVO reckon their electronic brakes are crap,had an F88 once seems nothing has changed.

19

Armagedon that reminds me of Mercs ill fated 10% (or whatever rate it was) trailer brake application when the exhauster was used, around the time of 1626’s.

This device was blamed on a fatal accident around 30 years ago, i well remember being in for MOT one day at the time, the tester removed my service line and told me to use my exhauster, if any air had come down the service line that was an immediate fail.

Dunno about you, but the last thing i want on a lorry is electronic brakes, they can stick the electric parking brake up their collective arses too.

20

In the winter you of course load the truck as heavy as just possible. 6x2 truck weighing 26t and just 10t on the triple will make the truck unstoppable and a dream to drive.
Summer time on the other hand I actually try to get more weight on the triple than on the truck since it tends to roll much easier = saving fuel

But the hole idea of a 4x2 truck pulling a trailer with 3 wheels almost in the center is just stupid and plain right dangerous in every other condition than dry summer. Can’t understand how EU allows it.

When loading max on the other hand you have the thing of getting absolutely perfect. Happens just 1 in 10 so had to take a pic. (26t on the tripple is 23t in real life so I’m legal over here)